1. Field of the Invention
The present invention relates to fabric stretchers and more particularly to an apparatus for tensioning and re-tensioning screen fabric for use in screen printing processing or the like.
2. Description of the Prior Art
Stretching frames are used for a wide variety of applications. One prevalent use for such frames is in the field of graphic arts. In the medium of oil painting, the canvass must be stretched into a taught plane so the paint may be applied onto a firm and stationary surface. Frequently the frame used for such art is made of wood and the canvass is attached to one side and then pulled across to the opposite side where it is anchored. Such frames may be fashioned so that a wedge may be driven into each of the corners of the frame to provide tension for the tightening of the canvas.
Printing screens may be used for transfer of inks for the creation of an art work or in the printing industry. Screen mesh as used in the screen print industry is manufactured from polyester and is typically supported and tensioned between opposing sides of a four sided frame. The mesh, when uniformly stretched in a taught plane, provides the foundation for a stencil. Printing ink is then forced through the open areas of the stencil by means of a blade or squeegee creating an image in the shape of the stencil on the work piece positioned beneath the screen. To achieve optimum results, several factors must be considered. The screen must be maintained at the recommended working tension throughout the print run so the pattern is transferred with accuracy. Tension variation must be kept to a minimum. Off contact must be kept to a minimum.
During the printing stroke, the squeegee blade deflects the fabric downwards forcing it into direct contact with the work piece during its passage across the screen. This deflection causes the fabric to be stretched from its undisturbed condition. The constant cycle of stretching and snapping back accelerates the deterioration of both the screen and the stencil. Proper tensioning decreases off contact, improves image quality and provides longer stencil and image life. Additionally, as the screen mesh undergoes these distorting forces during the print stroke, the stencil in the screen may also be distorted, especially if the contact between the screen and the fabric is redundant. This can result in a slightly distorted image on the work piece.
Typical tubular alloy roller frames used to stretch the screen mesh utilize tubes positioned on all four sides of the frame. The screen is pulled over these tubes and locked in place. In doing so, the frame may become slightly distorted and this distortion can cause the off contact distance between the screen and the work piece to vary across the print area with a consequent reduction in print quality and often they require rigidizing trusses to correct.
The majority of screen frames currently available do not allow the mesh and stencil to be easily removed and reinstalled on the frame. Consequently screens must be stored mounted on their frames between print runs. Removal of the mesh and accurate reinstallation for later use represents an improvement over the prior art. Applicant""s print screen frame tensioning apparatus addresses these problems and represents a novel contribution to the screen printing industry.
An object of the present invention is to provide for a novel print screen frame tensioning apparatus which allows for the accurate tensioning of the screen.
A further object of the present invention is to provide for a novel print screen frame tensioning apparatus which is easily assembled and has few moving parts.
A still further object of the present invention is to provide for a novel print screen frame tensioning system which allows for the screen to be stored for reuse without the necessity of maintaining the screen on the frame.
A still further object of the present invention is to provide a classical cross sectionally square or rectangular rails which are easier to work with and provide superior sealing of the ink well.
A still further object of the present invention is to provide for a novel print screen frame tensioning apparatus which maintains its square shape during tensioning.
The print screen frame tensioning system, generally rectangular in shape and fabricated from extruded rectangular tubular rails, when assembled, each side rail of the frame has a longitudinal one way channel, circular in cross sectional area, formed in each of the tubular rails for substantially the length of the tubular rails for receipt of a locking rod, the locking rod being axially serrated and the longitudinal channel being axially serrated, each locking rod having an additional tensioning fitting formed on one of its longitudinal ends for lateral tensioning, the frame having a plurality of throughbores in its side walls for receipt of a plurality of locking bolts, the frame and print screen being assembled by placing the frame on a support, spreading the screen or fabric material across the frame, positioning the locking rods in their respective longitudinal channels in their respective tubular rails, selectively engaging the end of the lock rods with a wrench to rotate them and thus cause tension vertically on the screen or fabric panel, and simultaneously tightening the plurality of locking bolts in selective throughbores during the tensioning process. There are two dimensions necessary to achieve maximum uniform tension of the fabric panel, vertical and lateral. Vertical being the center point of each panel to the center of each rail and lateral being the center of each rail to both ends of each rail.